The present invention relates to calibrating an electronic printer output device. It finds particular application in conjunction with calibrating an electronic printer output device using a tone reproduction curve (“TRC”) and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other like applications.
An electronic printer output device (“printing device”), also referred to as an image output terminal (“IOT”), is generally capable of producing color and standard black-and-white tones to produce images. The printing device accepts color level specifications (typically for each of four (4) colors including cyan, magenta, yellow, and black (“CMYK”)) as input and produces corresponding color areas on a printed page. This production of the corresponding color areas is often performed by a screening operation (also referred to as a halftoning operation) wherein a fine pattern of color dots is printed. The dots appear as varying color tones in accordance with the number of color dots used when viewed from a distance. The allowed color level specifications for each of the CMYK color spaces generally vary over some finite range such as 0 to 1 or 0 to 255. The colors at the extremes (i.e., 0 and 1 or 0 and 255) are white and saturated CMYK colors, and numbers between the extremes yield intermediate color tones. However, while the color level specifications may vary linearly, a linear change in color levels typically does not result. Instead, there may be a particular threshold before lighter toned colors become visible. Similarly, darker toned colors may prematurely appear saturated.
A printing device is designed to operate under certain conditions. More specifically, a printing device is calibrated to operate at an ideal set-point, for producing relatively consistent output when used within an environment having specific desired operating parameters such as, for example, a specific humidity, temperature, and dust count, etc. If any of the operating parameters deviates from the desired operating parameter, the printing device may drift away from the ideal set-point. To some extent the drift can be corrected with controlled feedback mechanisms within the printing device itself, but typically requires further corrective action to maintain color consistency in the printed output. This invention relates to this corrective action.
One way to compensate for the drift within the printing device includes modifying data within a printing device output device controller (“controller”). The controller converts image data (e.g., PostScript® data or PDF data) into raster data, which is transmitted to the printing device where it is printed. More specifically, a calibration (or correction) TRC modifies the raster data before it is sent to the printing device. The resulting image produced by the printing device using the corrected raster data is printed correctly (i.e., as if the printing device had not drifted from the ideal set-point and the data was not corrected by the TRC). Both linear and nonlinear drifts within a printing device may be compensated by using calibration TRCs.
In order to accurately produce a desired color level, the correspondence between the color level specification of the printing device and the actual color level produced by the printing device must be known. The TRC provides an illustration of the corresponding relationship.
In order to determine the TRC, a sample set of color level specifications are printed and the color levels actually produced by the printing device are measured by a device such as a calorimeter or spectrophotometer. The measured points are then used as raw data to calculate a set of curves. The set of curves produced are referred to as the calibration TRCs, or simply as TRCs. Various methods exist for creating TRCs. One such method is set forth in U.S. Pat. No. 5,696,889, which is hereby incorporated by reference.
Although TRCs can be created and applied to correct for drifts in a printing device output, there is currently no method or system that permits a user to specify a boundary (e.g., page, set, or job) after which a calibration process begins for a specified output medium. Furthermore, there is no method or system that permits the user to then specify a boundary (e.g., page, set, or job) after which TRCs that were calculated during the calibration process will be applied to future outputs of the printing device.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.